Magnesium alloy

ABSTRACT

The invention relates to the development of a corrosion-resistant magnesium secondary alloy. In the field of magnesium metallurgy, no secondary alloys have existed so far such as they exist in the case of other metals such as aluminium, for example. Magnesium alloys are corrosion-resistant if the copper, nickel and iron contents are very low. Recycling of old scrap metal with the aim of again producing structural parts has therefore not been possible up till now since too much copper and nickel are contained in this scrap metal According to the invention, this problem is solved by a new magnesium secondary alloy having been developed which, in spite of higher contents of copper, nickel, iron and silicon, possesses the same corrosion properties as pure magnesium alloys.

The invention relates to a corrosion-resistant magnesium alloy, in particular a secondary alloy melted from scrap metal, to its use and to a process for its production.

In the field of magnesium metallurgy, no secondary alloys have existed so far such as they exist in the case of other metals such as in the field of aluminium metallurgy, for example. Secondary alloys are smelted from scrap metal. They are then reprocessed into products.

It is known that magnesium alloys are corrosion-resistant if the copper, nickel and iron contents are very low. In the widely used alloys of the groups AZ (magnesium with aluminium and zinc), AM (magnesium with aluminium and manganese), AS (magnesium with aluminium and silicon) and AJ (magnesium with aluminium and silicon) and AJ (magnesium with aluminium and strontium) the limits of tolerance are usually fixed at 250 ppm copper, 10 ppm nickel and 50 ppm iron. If these contents are exceeded, strong corrosion, particularly by pitting, occurs, as indicated by Bakke, P., Sannes, S., Albright, D.: Soc. Automotive Engineers, paper 199901-0926, (1999), pp. 1-10 and Kammer, C. (editor): Magnesiumtaschenbuch (Magnesium handbook), Aluminium-Verlag Düsseldorf, 2000, 1^(st) edition, ISBN 3-87017-264-9. As a result, non-coated parts such as gearboxes or crank cases of magnesium alloys become unfit for use.

Coated parts, such as mobile phone, computer or chainsaw cases are exposed to attack by corrosion if the surfaces exhibit even slight damage. The magnesium alloys must consequently be produced from pure precursor products which must be free, above all, from copper and nickel since these elements cannot be removed from magnesium and its alloys without major effort.

The need to save energy and protect the environment requires recycling of products consisting of magnesium alloys. This becomes obvious if the energy expenditure of 35 kWh/kg of magnesium (disregarding the degrees of effectiveness for energy conversion) during the primary production of magnesium is compared with the energy requirement of 1 kWh/kg for recycling new scrap metal.

Recycling of old scrap metal and in particular of shredder fractions by simple re-smelting with an advantageous energy requirement that is only insignificantly higher than for fresh scrap metal has so far not been possible. Shredder fractions always contain scrap aluminium and consequently also copper. Even dismantled parts sorted according to type cause the same problems since nickel and copper pass from the coatings into the re-smelted alloys. As a result of this situation, no secondary alloys exist, contrary to the case of aluminium alloys which have been accepted for some considerable time. Known magnesium alloys with increased contents of aluminium and zinc are listed in table 1.

All the percentage data used in the following mean percentages by mass as is common practice in this field of specialisation.

TABLE 1 Composition of magnesium alloys with increased aluminium and zinc contents, in percent by mass, ASM Handbook, ISBN 0-87170-657-1, 199. max. max. max. max. max. Alloy Al Zn Mn Cu Ni Fe Si others AZ92 8.3-9.7 1.6-2.4  0.1-0.35 0.35 0.01 0.02 0.3 0.3 AZ125 11-13 4.5-5.5 0.3 AM100  9.3-10.7 0.3 0.1-0.5 0.1 0.01 0.023 0.18 0.3 AM90   7-9.5 0.3-2   0.15 0.35 0.02 0.05 0.5 (SIS144640)

The alloys AZ92, AM100 and AM90 from Table 1 have either high aluminium contents and low zinc contents or vice versa. With increased contents of copper, nickel and iron, the corrosion properties of these alloys are poor in comparison with a pure alloy as described below by way the figures and examples. Although the alloy AZ125 has high contents of aluminium and zinc, the sum total of the other components amounts to only 0.3%. However, a secondary alloy ought to be characterised by higher defined contents of copper, nickel and iron which are always present in secondary alloys, being tolerable regarding the corrosion resistance. Consequently, these alloys cannot be regarded as secondary alloys.

The invention was based on the object of developing a magnesium alloy which, in spite of higher defined contents of copper and nickel, possesses corrosion properties comparable with or better than a highly pure magnesium primary alloy.

Such a magnesium alloy can be obtained from scrap metal or impure precursors containing in particular copper or nickel by adjusting the components during smelting and be reused for structural parts.

The magnesium alloy according to the invention contains 10 to 20% by mass of aluminium, 2.5 to 10% by mass of zinc, 0.1 to 2% by mass of manganese, 0.3 to 2% by mass of copper or up to 2% by mass, preferably 0.001 to 2% by mass of nickel, essentially made up to 100% with magnesium, i.e., the alloy consists essentially of the above-mentioned components, it being possible for the fluxes mentioned in the sub-claims and, optionally, (further) impurities to be present in a small quantity.

In most cases, at least one of the elements of copper, nickel, cobalt, iron, silicon, zirconium and beryllium will additionally be contained in the alloy to a total content of up to 2% (copper in addition to nickel or nickel in addition to copper). It will be possible to speak of the element concerned “being contained” as a rule if it is present in a minimum quantity of approximately 0.001% by mass.

Surprisingly enough, it has been found that, in spite of frequently higher contents of copper, nickel, iron and silicon in the new magnesium secondary alloy in comparison with the limit contents of the alloys previously used for structural parts, the corrosion behaviour is just as good as that of the highly pure alloys.

Preferably, the impurity-tolerant magnesium alloy consists of 11 to 18% by mass aluminium, preferably 12 to 16% by mass aluminium, 3 to 8% by mass zinc, preferably 3 to 5% by mass zinc, 0.3 to 1.5% by mass manganese, preferably 0.5 to 1% by mass manganese, 0.3 to 2% by mass copper, preferably 0.45 to 0.8% by mass copper and, if necessary, at least one of the elements of nickel, cobalt, iron, silicon, zirconium and beryllium to a total content of 1.5%, preferably a total content of up to 1%, the remainder being magnesium.

The subject matter of the invention is consequently a magnesium alloy which contains the indicated contents of aluminium, zinc and manganese as well as additionally 0.3% to 2% copper and/or the elements nickel, cobalt, iron, silicon, zirconium and beryllium to a total content of up to 2%, preferably up to 1.5%, further preferably up to 1%, the elements of copper, nickel, cobalt, iron and silicon generally being introduced into the alloy by contaminated alloy starting materials and/or scrap metal.

Additional small contents of other elements which the expert would not consider to be alloy components may be present. Such contaminants are present within orders of magnitude of up to maximum 0.1% and in particular maximum 0.01%. Contaminants of this order of magnitude may in turn have been entrained by the use of impure precursor materials or scrap metal.

The nickel content of the magnesium alloy is preferably at least 0.001%, further preferably at least 0.003%. These nickel contents can be balanced by higher aluminium, zinc and manganese contents in the inventive alloys in the sense that, in spite of the higher nickel content, it has not been possible to detect increased corrosion properties.

Moreover, the magnesium alloy preferably contains at least 0.4% copper.

In a preferred further development of the invention, the magnesium alloy moreover contains up to 2% of at least one of the elements of calcium and strontium and in a further preferred embodiment up to 2% of at least one of the elements of the group of elements of rare earths, yttrium and scandium. According to a further embodiment of the invention, the magnesium alloy preferably contains up to 2% and further preferably at least 0.1% of cerium mixed metal. Cerium mixed metal is available commercially and well known to the expert. A typical composition for cerium mixed metal would be for example: rare earths at least 99.00% cerium maximum 57.12%, lanthanum maximum 36.19%, praseodymium maximum 4.33%, neodymium maximum 2.36%, iron maximum 0.54%, magnesium maximum 0.14%, silicon maximum 0.051%, sulphur maximum 0.01%, phosphorus maximum 0.01% (from Handbook of Extractive Metallurgy, vol. III, 1997).

Surprisingly enough it has been found that by adding strontium, calcium, rare earths, yttrium and scandium individually or in mixture in contents of up to 2%, the corrosion properties were further improved. The rates of corrosion of these alloys, determined according to the salt spray test according to DIN 50021, are indicated below in Table 4, the compositions in Table 5.

The magnesium alloy according to this invention is characterised preferably by the beta-phase having a network structure.

The magnesium alloy obtained preferably has a corrosion rate of less than 1.2 mm/year, measured by means of a salt spray test according to DIN 50021, as indicated below in further detail in the examples.

The object of the invention is also achieved by the magnesium alloy being a secondary alloy which was obtained by smelting scrap metal or impure precursor materials containing copper and/or nickel.

Such a magnesium secondary alloy can be produced in a cost-effective manner and is particularly suitable for the production of structural parts.

Such a magnesium alloy is also particularly suitable for the production and use of corrosion protection anodes in the fresh water sector.

The invention comprises also a process for the production of a magnesium alloy from precursor materials contaminated with copper and/or nickel, in particular from scrap magnesium, which process is characterised in that the scrap metal or the impure precursor materials are smelted and the alloy is adjusted to a content of components corresponding to an inventive magnesium alloy as described above.

The comparative corrosion investigations were carried out by immersion in a 3.5 and 5% sodium chloride solution and according to the salt spray test according to DIN 50021. During the immersion measurements, the rate of corrosion was determined by measuring the amount of hydrogen developed and/or by titration with hydrochloric acid. In the salt spray test, the loss of mass is determined. In Table 2, the rates of corrosion of a fresh secondary alloy, a pure alloy and a comparative alloy with similar contents of copper and nickel are compared. The composition of the alloys listed in Table 2 is given in Table 3.

TABLE 2 Rates of corrosion of the magnesium alloys in mm/year Immersion pH = 6 Salt spray test mm/year mm/year New secondary alloy 6.57 1.00 Pure alloy 5.73 1.07 Comparative alloy 35.27 23.79

TABLE 3 Composition of the alloys indicated in Table 2, in percent by mass Al Zn Mn Cu Ni Fe Si New 11.7 3.04 0.48 0.47 0.0032 0.0087 0.39 secondary alloy Pure alloy 8.65 0.67 0.20 0.0081 0.00061 0.0022 0.054 Comparative 8.17 2.84 0.21 0.0085 0.0026 0.023 0.18 alloy

The conditions for the immersion test were as follows

3.5% by weight of aqueous NaCl solution pH=6 (constant) Volume of the solution: 1.9 l Sample size: Diameter 25 mm, thickness 4 mm Sample treatment: ground with granulation 1200, rinsed with water and ethanol.

After immersion for at least 100 hours, a rate of corrosion of less than 10 mm/year is obtained for the alloys according to the invention and after at least 400 hours, a rate of corrosion of less than 20 mm/year. The salt spray test according to DIN 50021 showed that the magnesium alloys according to the invention exhibit rates of corrosion of less than 1.2 mm/year and consequently the highly pure magnesium alloys are at least comparable.

The microstructure of the new secondary alloy is determined by a very small grain size and a change in the beta phase Mg₁₇Al₁₂. In this case, the beta phase forms a network structure according to FIG. 1, which slows down the corrosion attack caused by the local element generators copper, nickel, cobalt and iron. The microstructure of the pure alloy AZ91, on the other hand, does not exhibit the network structure of the beta phase, FIG. 2. The new alloy is consequently tolerant vis-à-vis high contents of copper, nickel, cobalt and iron.

In FIG. 3 and Table 2, an alloy is also given which shows how a contaminated alloy with only a slightly raised nickel and iron content is corroded when the composition does not correspond to a secondary alloy according to the invention. This alloy is referred to as “comparative alloy”.

TABLE 4 Rates of corrosion of the magnesium alloys with strontium, calcium and rare earths, in mm/year. Magnesium secondary alloy Salt spray test mm/year With strontium 0.67 With calcium 0.25 With rare earths 0.33

TABLE 5 Composition of the alloys indicated in Table 4, in percent by mass Magnesium secondary alloy Al Zn Mn Cu Ni Fe Si With 0.0034% 10.97 3.28 0.61 0.47 0.0037 0.0043 0.12 strontium With 0.30% 9.84 2.38 0.40 0.27 0.0025 0.0014 0.10 calcium With rare earths*) 10.47 3.00 0.60 0.47 0.0025 0.0047 0.15 *)0.16% Ce, 0.13% La, 0.028% Pr, 0.039 Nd

The magnesium secondary alloys according to the invention are excellently suitable for use as structural parts in the cast alloy field and the semi-solid casting process, e.g. New Rheo Casting. They are also suitable for use as corrosion protection anodes in the fresh water field.

The invention improves the state of the art regarding the following aspects:

-   -   secondary alloys for structural parts can be introduced on the         market which are more cost effective than the highly pure         magnesium alloys and which have the same rate of corrosion.     -   the new alloys permit the cost and energy effective recycling of         old scrap metal with the aim of producing new structural parts     -   more impure precursor materials containing copper and nickel can         be used for the new alloys. Consequently, refining steps can be         omitted during the production of magnesium oxide and magnesium         chloride.     -   downcycling such as the use of shredder material in the         aluminium sector or as desulphurising agent in the steel         industry is avoided.

The invention will be explained in further detail by way of practical examples.

EXAMPLE 1

A magnesium alloy with the composition 11.7% Al, 3.04% Zn, 0.48% Mn, 0.47% Cu, 0.0032% Ni, 0.0087% Fe and 0.39% Si in the alloy is smelted. The production of the alloy took place at 760° C. The melt was cast into a copper chill mould preheated to 200° C. In the same way, a highly pure reference alloy with the composition of 8.65% Al, 0.67% Zn, 0.20% Mn, 0.0081% Cu, 0.00061% Ni, 00022% Fe and 0.054% Si was produced. The reference alloy corresponds to a highly pure magnesium alloy AZ 91 with a particularly high resistance to corrosion.

From the alloy rod, discs with a diameter of 25 mm and a thickness of 4 mm were cut and subjected to the corrosion test. The corrosion test was carried out by immersing the discs into a 3.5% aqueous NaCl solution at a constant pH. According to the reaction

Mg+2H₂O=Mg(OH)₂+H₂

one mol of hydrogen is formed per atom of magnesium. As a result, the rate of corrosion can be determined from the volume of hydrogen formed. The corrosion behaviour of the new secondary alloy is compared in FIG. 3 with the reference alloy as well as a comparative alloy contaminated with Ni and Fe and having the composition 8.17% Al, 2.84% Zn, 0.21% Mn, 0.0085% Cu, 0.0026% Ni, 0.023% Fe and 0.18% Si. FIG. 3 shows the hydrogen development in 3.5% NaCl solution at a constant pH=6. After a varying initial hydrogen development in FIG. 3, the corrosion behaviour is characterised by the linear regions in FIG. 3. The rates of corrosion calculated therefrom are indicated in Table 2. In Table 2, the rates of corrosion determined from the salt spray test according to DIN 50021 are also recorded.

From Table 2 it can be seen that the new secondary alloy has the same rates of corrosion as the highly pure alloy.

EXAMPLE 2

A magnesium secondary alloy with the composition of 9.84% aluminium, 2.38% zinc, 0.40% manganese, 0.27% copper, 0.0025% nickel, 0.0014% iron, 0.10% silicon and 0.30% calcium, the remainder being magnesium, was smelted. The rate of corrosion, determined by the salt spray test according to DIN 50021, amounted to 0.25 mm/year. The rate of corrosion is consequently considerably below that of the highly pure alloy AZ 91 of 1.07 mm/year.

EXAMPLE 3

A magnesium secondary alloy with the composition of 10.97% aluminium, 3.28% zinc, 0.61% manganese, 0.47% copper, 0.0037% nickel, 0.0043% iron, 0.12% silicon and 0.0034% strontium the remainder being magnesium, was smelted. The rate of corrosion, determined by the salt spray test according to DIN 50021 amounted to 0.67 mm/year. The rate of corrosion is consequently considerably below that of the highly pure alloy AZ 91 of 1.07 mm/year.

EXAMPLE 4

A magnesium secondary alloy was smelted with an addition of cerium mixed metal with the composition of 10.47% aluminium, 3.00% zinc, 0.60% manganese, 0.47% copper, 0.0025% nicked, 0.0047% iron, 0.15% silicon and 0.16% cerium, 0.13% lanthanum, 0.028% praseodymium and 0.039% neodymium, the remainder being magnesium. The rate of corrosion, determined by the salt spray test according to DIN 50021, amounted to 0.33 mm/year. The rate of corrosion is consequently considerably below that of the highly pure alloy AZ 91 of 1.07 mm/year. 

1. Magnesium alloy containing 10 to 20% by mass of aluminium, 2.5 to 10% by mass of zinc, 0.1 to 2% by mass of manganese, 0.3 to 2% by mass d˜ copper or up to 2% by mass, preferably 0.001 to 2% by mass of nickel, made up essentially to 100% with magnesium, i.e., the alloy consists essentially of the above.
 2. Magnesium alloy according to claim 1 characterised in that additionally, at least one of the elements of copper, nickel, cobalt, iron, silicon, zirconium and beryllium is contained therein up to a total content of 2% by mass
 3. Magnesium alloy according to claim 1 consisting of 11 to 18% aluminium, preferably 12 to 16% aluminium, 3 to 8% zinc, preferably 3 to 5% zinc, 0.3 to 1.5% manganese, preferably 0.5 to 1% manganese, 0.3 to 2% copper, preferably 0.45 to 0.8% copper and, optionally, at least one of the elements of nickel, cobalt, iron, silicon, zirconium and beryllium to a total content of 1.5%, preferably a total content of up to 1%, the remainder being magnesium (% by mass respectively).
 4. Magnesium alloy according to claim 1 characterised in that the nickel content amounts to at least 0.001%, preferably at least 0.003%.
 5. Magnesium alloy according to claim 1 characterised in that the copper content amounts to at least 0.4%.
 6. Magnesium alloy according to claim 1, containing up to 2% of at least one of the elements of calcium and strontium.
 7. Magnesium alloy according to claim 1 containing up to 2% of at least one of the elements from the group elements of rare earths, yttrium and scandium.
 8. Magnesium alloy according to claim 1 containing up to 2% and preferably at least 0.1% of cerium mixed metal.
 9. Magnesium alloy according to claim 1 characterised in that the beta phase exhibits a network structure.
 10. Magnesium alloy according to claim 1 characterised in that it possesses a rate of corrosion of less than 1.2 mm/year measured by means of a salt spray test according to DIN
 50021. 11. Magnesium alloy according to claim 1 characterised in that it is a secondary alloy which has been obtained by smelting scrap metal or impure precursor materials containing copper and/or nickel.
 12. Use of the magnesium alloy according to claim 1 for the production of structural parts.
 13. Use of the magnesium alloy according to claim 1 for the production of corrosion protection anodes.
 14. (canceled) 